1. Field of the Invention
The present invention relates to a semiconductor wafer mapping apparatus. More particularly, the present invention relates to a semiconductor wafer cassette mapper for detecting number and position of wafers in a wafer cassette.
2. Description of the Related Art
In order to reduce dust and contamination from a manufacturing process, an automatic mechanism is required to perform automatic processes. When a wafer cassette holding a number of wafers is placed in an appropriate position, a robotic arm transports the wafers between a wafer processing chamber and the wafer cassette. Several types of wafer cassettes are used to hold these wafers, depending on the particular process being performed. For example, Teflon wafer cassettes are employed to transport wafers while they are in chemical processes, and quartz wafer cassettes are employed to hold wafers during high temperature processes. In between these processing steps, automated transfer devices are used to transfer the wafers from one type of wafer cassette to another.
FIG. 1 is a schematic, three-dimensional illustration of the conventional semiconductor wafer cassette mapper.
Referring to FIG. 1, a semiconductor wafer cassette 100, capable of holding a full complement of twenty-five wafers 101, is shown. Each of the wafers 101 is disposed horizontally in evenly-spaced slots 103 located on each sidewall of wafer cassette 100.
While performing the semiconductor process, number and position of the wafers in the wafer cassette depend on the process performed on the wafers. A semiconductor wafer cassette mapper for detecting number and position of wafers in a wafer cassette is required to enable a robotic arm to accurately remove and place wafers in the wafer cassette. Conventionally, an emitter apparatus is located at one side of wafer cassette 100 and a receiver apparatus is located at the opposite side thereof. The emitter apparatus is leveled with the receiver apparatus, and the wafer cassette 100 is stationary between the emitter apparatus and the receiver apparatus. The emitter apparatus and the receiver apparatus move up and down simultaneously at a constant rate to scan the wafers 101 (or the emitter apparatus and the receiver apparatus are stationary, and the wafer cassette moves up and down at a constant rate). The presence or absence of a wafer 101 in the wafer cassette 100 is determined by whether a beam from the emitter apparatus is detected by the receiver, or not. Moreover, a double slot effect that multiple wafers 101 are stacked or a cross slot effect that a wafer 101 is not placed horizontally can probably be determined by the wafer cassette mapper.
The position of the wafers is scanned by moving either the wafer cassette, or the emitter and the receiver. However, no matter which mechanism is used, the speed of moving elements must be controlled accurately to prevent deviation of mapping or sway of the wafers in the wafer cassette. As a result, a complex and accurate driving system is used. FIG. 2 is a schematic, three-dimensional illustration of the conventional driving system for controlling a wafer cassette moving up or down. These complex parts are not described herein since they are not closely related to the scope of the present invention. The complicate mechanism fails easily, causing equipment shutdown or incorrect mapping results. For preventing deviation of mapping or sway of the wafers under mapping process, it is necessary to control cassette a slow travelling rate, thus time of wafer mapping is further increased and wafer throughput is decreased.